Structural and functional unit of life – The Cell

Our earth is a wonderful place where many different kinds of organisms co-exist. The diversity observed among living forms is huge- ranging from minute Algae to tall trees, micro organisms like bacteria to gigantic Blue whales. The thing that unifies all of them is that all have a basic unit of life called as “Cells”. A Cell is the smallest functional unit of any living organism on this planet. Human body is made of trillions of these tiny structures which are capable of performing independent function and also in association with other cell types or tissues of our body.

Structure of Cell:

As our body has many organs to perform specific functions, the cell also has something similar called as “Organelles”. These organelles perform specific independent functions in a cell and also work in combination with other organelles of the cell, which eventually make the cell a living entity.

All the cells in our body share some important features in spite of exhibiting different morphologies:

1. They are covered by a outer membrane called as Plasma membrane or Cell membrane.
2. The cell is filled with a viscous fluid called as Cytoplasm.
3. Many organelles are present floating inside the cell’s cytoplasm.
4. Most of the cells have a central control zone called as Nucleus.

Plasma membrane: It is a flexible membrane and made up of organic molecules called Lipids (fats) and proteins. This membrane separates the contents of the cell from the extra cellular fluid present outside of the cell. The Plasma membrane also acts a barrier to allow or exit of some molecules in or out of the cell. Hence cell or plasma membrane is also called as semi or selectively permeable membrane.

Cytoplasm: A jelly-like fluid in which majority of the functions of the cell are carried out. Many cell organelles are present floating inside this viscous fluid. Cytoplasm is constituted by all cell organelles, cytosol (cytoplasmic matrix or fluid without organelles and their contents) along with many proteins, lipids, carbohydrates and ions present in cytoplasmic matrix. Different organelles in the cell can perform unique functions and many are separated from the cytosol by membranes (such as Nucleus, mitochondria, Golgi, ER, peroxisomes etc.). In short we can say, cytoplasm directs all life processes of cell.

Nucleus : The nucleus is the cell’s control center as it contains the genetic material “Deoxyribo nucleic Acid” (DNA). The nucleus is covered by a double membrane called as nuclear envelop and this separates nucleus from the cytoplasm. The nuclear membrane consists of pores in the membrane which allows transport of material from inside of the nucleus to outside in cytoplasm. As mentioned, nucleus contains DNA which are organized in the form of chromosomes. These chromosomes are visible as rod-like structures only when cells are about to divide. Chromosomes are made up of DNA and proteins called Histones. These proteins help to fold DNA in a firm manner and also gives the chromosome a more compact shape, so as to fit the long strands of DNA into tiny nucleus compartment.

Mitochondria: They are sausage-shaped organelles bound a double membrane and also contains their own genetic material (DNA) and ribosomes. Mitochondria is considered as power house of the cell as they are sites for release of energy in the form of adenosine Tri phosphate (ATP) from glucose and other foods.

Rough endoplasmic reticulum: They resemble a network of tubes and bags attached to the nuclear envelop. Proteins and other molecules move through the endoplasmic reticulum between various regions of cytoplasm and also between cytoplasm and nucleus. If the outer surface of endoplasmic reticulum (ER) is filled with Ribosomes (another cell organelle required for protein synthesis), it is called Rough ER. Smooth ER lacks ribosomes and are known for synthesis of lipids and carbohydrates which can used for building cell membrane. Some other lipids and proteins function as hormones and enzymes.

Ribosome : These tiny structures are considered as factories of the cell, where proteins necessary for building and running the cell are produced. These ribosomes are present on Rough endoplasmic reticulum and also freely in cytoplasm. They link amino acids together to form proteins based on information encoded in DNA.

Golgi apparatus: The Golgi apparatus consists of membrane-bound vesicles arranged parallel to each other in stacks called as cisterns. These membranes always have some connections with the ER and this allows them to package and dispatch proteins and other materials synthesized in ER to various destinations, inside and outside the cell. The other functions of these vesicles include storage, modifications and package of ready to use materials required for proper functioning of cells.

Lysosomes: These are membrane bound sac like structures that contains enzymes required to breakdown and digest any unwanted material or worn out organelles. Lysosomes are a kind of waste disposal system of the cell. They tend to keep clean by digesting any in coming foreign organisms like bacteria. The powerful enzymes synthesized in RER which is capable of breaking down any organic molecule allows lysosomes to perform their function efficiently. In case of any disturbances in cell metabolism or cells get damaged, lysosomes burst open to release the enzymes to digest own cell. Hence, Lysosomes are also called as “suicide bags of the cell”.

Vacuoles: They are small sacs like structures used for storing food or liquid contents.

Microtubule, Microfilaments and Intermediate fibers: These are fibers of cytoskeleton necessary to provide the shape to the cell (internal scaffolding) and help in movement of organelles. Microtubules are made up of Tubulin, microfilament is made of actin and Keratin makes intermediate fibers.

Peroxisome: They are also membrane bound structures which contain enzymes that break down fatty acids and some chemical toxins.

Centrioles: Found in centrosomes and essential in organizing DNA movement during cell division.

Further Reading :

Plasma membrane : Detailed structure and function of plasma membrane

Image Credit: The entire cell structure was assembled using individual components from Smart.servier.com. The image of the ultrastructure of cell can be re-used in any manner ( A link to this blog post is appreciated -Thanks)

Centrioles image : OpenStax.org